This invention relates to a method for controlling a photovoltaic inverter for the ride-through of a network fault and a photovoltaic inverter system.
Decentralized energy generation plants, photovoltaic (PV) plants, are increasingly being used to feed energy into public or private supply networks. PV plants use inverters to convert DC power generated by a PV generator into AC power. In the case of network coupling, the inverters operate on the output side phase-synchronously with the network so that they generate an AC voltage corresponding to the frequency and amplitude of the network voltage and a suitable AC current. Inverters are known in various topologies with half-bridge or full-bridge circuits which include pulsable semiconductor switching elements, generally power MOSFETs, IGBTs, etc., which are suitably controlled at high frequencies in order to produce the required AC current of the desired phase and amplitude.
In order to obtain the highest possible yield, a PV generator is operated at a so-called maximum power point (MPP), which is the point in the current-voltage diagram of the PV generator at which the greatest power can be drawn, i.e. at which the product of the current and voltage is maximized. The MPP operating point is not constant, but rather depends on the irradiance, the temperature, the type of solar cells and other factors. In a PV inverter, the MPP operating point is often set by a so-called MPP tracker which adjusts the voltage of the PV generator to a suitable value.
As ever more and ever larger-sized PV energy generation plants are put into operation and connected to supply network, many network operators and countries are demanding that PV generators remain connected to the electrical power supply network from a specific minimum power in the event of small controllable network faults and continue to supply power to the network, in order to prevent an unintended simultaneous shutdown of the feed-in powers and thus breakdowns of the entire network. For example, the Medium-Voltage Directive in Germany requires that energy generation plants, including PV plants, with more than 100 kW peak power, which feed their power into the medium-voltage network, must, in the event of a short-circuit in the network, remain connected to the network and must make a specific short-circuit current available. This is known as fault ride-through (FRT) and is also referred to as low-voltage ride-through (LVRT) or zero-voltage ride-through (ZVRT). According to the German Medium-Voltage Directive, a specific reactive current must be fed as a short-circuit current of around 90% of the nominal current in the case of voltage dips of 50%. Similar feed-in directives also exist in other countries around the world.
There is a risk that an inverter system will be damaged due to an overvoltage in its DC circuit if it is connected to the network during an FRT. Since the PV generator continues to supply power to the DC voltage link, the DC link voltage can rise to values close to the open-circuit voltage of the PV generator if the AC voltage is reduced significantly during the FRT event. This increase in the DC link voltage can damage the power semiconductor switch used in the inverter if its nominal voltage values are exceeded. On the other hand, the use of sufficiently voltage-proof power semiconductor switches can be expensive.
The high increase in the DC link voltage leads to a natural limitation of the DC voltage due to the output characteristic curve of the PV generator, while at the same time also limiting the current supply capability of the inverter if its power semiconductor switch can no longer commutate the required short-circuit current at a high DC link voltage.
To overcome this problem, U.S. Pat. No. 8,687,328 B2 proposes using a crowbar known as a brake chopper to prevent an increase in the DC link voltage during an FRT event. This clamping circuit (the brake chopper) comprises a series circuit consisting of a controllable switch and a brake resistor which is connected to the DC voltage link parallel to the PV generator to dissipate energy from the PV generator and convert it into thermal energy in the brake resistor. Consequently, the DC link voltage can be clamped to a desired maximum value.
In particular, U.S. Pat. No. 8,687,328 B2 proposes, in the event of a network voltage dip, detecting the operating voltage of the PV generator directly before the occurrence of the network voltage dip and closing the switch of the clamping circuit when it is detected that the operating voltage of the PV generator exceeds a predetermined threshold value of the operating voltage of the PV generator directly before the network voltage dip, and opening the switch of the clamping circuit when it is detected that the operating voltage of the PV generator has fallen below the value of the operating voltage of the PV generator directly before the network voltage dip. The voltage across the link is thus maintained in a range between the voltage of the PV generator directly before the network voltage dip and the upper threshold value which is higher by a differential voltage AV, and generally fluctuates between these two limit values during the FRT event.
However, if the operating voltage of the PV generator directly before the network voltage dip is relatively low due to shadowing effects, low temperature, etc., it may happen that the activated clamping circuit is not capable of holding the increasing operating voltage of the PV generator. The operating voltage of the PV generator can then increase even though energy is dissipated from the link via the brake resistor of the clamping circuit and converted into thermal energy. This results in unnecessary energy losses.